Regulation and function of striatal nitric oxide producing interneurons

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The striatum is a subcortical nucleus that regulates a number of complex activities ranging from voluntary action selection to the subconscious formation of habit. The coordination of these operations is mediated by the principle cells of the striatum, spiny projection neurons (SPNs). SPN activity is dictated by a confluence of cortical and thalamic glutamatergic inputs, their intrinsic excitability, and striatal microcircuits to achieve a desirable outcome. Striatal microcircuitry is of particular interest as it has historically, been the most poorly described of these elements due to the inherent difficulty in studying a nucleus without a laminar organization. Consequently, our knowledge of the interneurons that compose these circuits is scant. The advent of transgenic mice and novel constructs allowing for the identification and selective excitation of these cells has provided the opportunity to attempt a comprehensive description of basic striatal physiology. Accordingly, the goal of this thesis work is to determine the function and regulation of one class of striatal interneuron known as the plateau and low threshold spiking interneurons (PLTSIs). We used a combination of optogenetics, pharmacology, and calcium imaging to test the hypothesis that these cells mediate a form of synaptic plasticity in SPNs. To investigate the regulation of PLTSIs we combined mapping studies using a monosynaptic Rabies virus (RV), optogenetics, and pharmacology. Glutamatergic inputs onto SPNs were found to undergo long term depression (LTD) when PLTSIs were optogenetically activated. One of many neuromodulators these cells synthesize is nitric oxide (NO). We found that this LTD was dependent on NO and occurred via a postsynaptic mechanism that relied on removal of AMPARs from postsynaptic synapses. What’s more, this form of LTD was distinct from the canonically described endocannabinoid (eCB) LTD. To move towards an understanding of what regulates NO release, we first asked what regulates PLTSI activity. RV mapping experiments revealed quantitatively similar input from cortical and thalamic nuclei whereas functional studies using optogenetics to selectively excite either region revealed differential responses in PLTSIs. Whereas cortical stimulation induced high-frequency spiking followed by a pause, thalamic activation halted PLTSI firing and was frequently followed by rebound bursting. The thalamic response was due to activation of a disynaptic circuit involving striatal cholinergic interneurons (ChIs) as the thalamic pause was sensitive to antagonists of muscarinic acetylcholine receptors (mAChRs); enhanced by inhibitors of ACh degrading enzymes; and mimicked by direct optogenetic activation of ChIs. The pause and pause-bursting responses were dependent on Gi-couple M4 receptors. Because both the ACh and NO systems are altered in many diseases (e.g. Parkinson’s disease, Levodopa-induced dyskinesia) our data suggests a potential role for PLTSIs and the PLTSI-ChI interaction in initiating or augmenting these states.

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  • 02/19/2018
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